Losing weight while firming up is considered the Holy Grail of fitness, and scientists say they've found a way to do just that. The only caveat: It's not exactly fun or sustainable in the long term. But it does offer good information about the interplay between diet and exercise, and what it means for weight loss.

A new study published in The American Journal of Clinical Nutrition had 40 men undergo a month of six-days-a-week workouts while eating 40 percent less calories than the recommended daily allowance based on their height and weight. For exercise, the men did resistance training and high-intensity interval training (HIIT). For food, half of the group was given a high-protein diet, while the rest were fed lower-protein diets.

At the end of the study, the high-protein group lost about 10.5 pounds while the low-protein group shed eight pounds. They all got stronger and were in better shape—but the high-protein group gained about 2.5 pounds of muscle while their low-protein counterparts gained none.

Clearly the study focused on men, but study co-author Stuart M. Phillips, Ph.D., director of the Physical Activity Centre for Excellence at McMaster University says he expects that women would have a similar outcome. In fact, he's done previous research that says as much. "I have reason to suspect that women wouldn't be any different," he says. "This kind of pattern of weight loss—all fat and lean gain—is very achievable in women."

Aside from the fact that you'd be really, really hungry on this kind of diet, women's health expert Jennifer Wider, M.D., points out that people on extreme calorie-restricted diets for four to 16 weeks report having fatigue, constipation, nausea, dizziness, and diarrhea. "Most nutritionists and health care practitioners would not recommend a diet this low in calories for any extended period of time," she says.

"This is not sustainable long-term," agrees Doug Sklar, a certified personal trainer and founder of New York City fitness training studio PhilanthroFIT. "It's not something that I would suggest for clients to follow." A large calorie deficit can also negatively affect hormones in women, he adds.

But experts say there is something to be said for the combination of cutting back on your caloric intake and working out regularly—you just don't need to be extreme about it. Instead, try cutting 150 to 200 calories out of your diet and see how you do.

And, since we know you're wondering, cutting calories while practicing intensity training won't make you bulk up. "In order to 'bulk up,' one must create a calorie surplus, the opposite of a deficit," says Sklar. "Going into a calorie deficit will not produce large bulky gains in females or males."

Phillips himself agrees that a program this extreme is not a long-term approach to weight loss. "It's too hard to handle for more than four to maybe six weeks," he says. "We think that a weight-loss protocol that is less severe is the key."